An oscillator is an electronic device that uses an amplifier with positive feedback to generate a signal. The output of the amplifier is fed back, in phase, to an input of the amplifier to regenerate and sustain the signal. Oscillators are employed in a wide array of devices, such as computers and wireless transceivers.
In a wireless transceiver, such as a Radio Frequency (RF) communications systems, one or more oscillators provide the signal to a transmitter for upconverting (modulating) to an RF signal and to a receiver for downconverting (demodulating) from an RF signal. Modulation accuracy of the transceiver is essential to minimize bit error rate (BER) during communications. This results in challenging noise requirements for the oscillator(s), including stringent specifications for close-in and far-out phase noise.
The majority of oscillators used in RF communications systems are implemented using a single inductor-capacitor (LC) resonant circuit (“tank”) with a single negative-resistance stage. Both single-ended, differential and quadrature oscillators are designed based on this principle for which the single LC tank provides only a second-order filtering to the noise power injected into the oscillator. In these oscillators with low order filtering, a low quality factor (Q) of the LC tank results in large power consumption and poor phase noise performance. Thus, a high LC tank Q, available headroom of a power supply and current consumption are needed for an oscillator to satisfy existing RF communications standards.
As a whole, however, the wireless communications industry is moving in the opposite direction regarding component Q as newer complementary metal-oxide semiconductor (CMOS) process technologies (also called process nodes) create products having an inferior LC tank Q and power supply operating margin. In addition, competition in the industry demands wireless products with lower current consumption so that a mobile terminal can last longer with present battery technology.
Thus, present oscillators typically require a high-Q inductor and a high-Q capacitor to meet stringent phase noise requirements demanded by the industry and communications standards. Future CMOS process nodes, however, will continue to decrease the Q of the LC tank resulting in phase noise worsening. Accordingly, additional fabrication costs will be required to produce high-Q tanks. In addition, increased quality demands for wireless applications place even more stringent phase noise requirements on the industry.
Accordingly, what is needed in the art is an oscillator having a low phase noise that satisfies wireless communications standards. More specifically, what is needed is an oscillator that can be implemented using present and future CMOS process technology that satisfies low phase noise requirements for stringent RF communications standards.